1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to encoding and decoding an image, and more particularly, to entropy encoding and entropy decoding residual values.
2. Description of the Related Art
In image compression methods such as moving picture experts group (MPEG)-1, MPEG-2, MPEG-4 H.264/MPEG-4 Advanced Video coding (AVC), a picture is divided into macro blocks in order to encode an image. When predictive blocks are generated using inter prediction or intra prediction, the predictive blocks are subtracted from original blocks, thereby generating residual blocks.
Residual values of the residual blocks are discrete cosine transformed (DCT) to be in the frequency domain and discrete cosine coefficients generated as a result of the transformation are quantized. The quantized discrete cosine coefficients are entropy encoded through binarization and arithmetic coding and image data is generated.
In such a method of encoding an image in block units, the image may be damaged during quantization of the discrete cosine coefficients. However, the damage of the image is accepted due to improved compressibility and the image is encoded using DCT and quantization.
In addition, in an image encoding method such as Lossless JPEG, JEPG-LS, or XENA, DCT and quantization are not used and instead, pixel values are directly entropy encoded. Each of the pixel values are predicted in order to generate predicted values and the predicted values are subtracted from the pixel values, thereby generating residual values with respect to each of the pixel values. Then, the generated residual values are entropy encoded. Entropy encoding of the residual values using context models determined with reference to the pixel value(s) that are encoded before current pixels shows high performance in terms of compression.